Heat engine and method of operation

ABSTRACT

A closed cycle heat engine is provided. The heat engine includes first and second expansion pistons that are fluidly coupled to a heater. The expansion pistons are also fluidly and operably coupled to a compression piston. A regenerator extracts heat from a working fluid flowing from the expansion cylinders to the compression cylinders to preheat the working fluid flowing to the heater. A cooler is arranged in between the regenerator and the compression cylinder to remove additional heat before the working fluid reaches the compression cylinder. The heat engine is arranged such that the working fluid travels unidirectionally within the engine. The heat engine may further include one or more actuated valves for controlling the flow of the working gas. In one embodiment, the pressure of the crankcase is controlled to be at or less than the heat engine minimum pressure.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Patent Application 61/147,493, filed Jan. 27, 2009 entitled “Heat Engine and Method of Operation”, which is hereby incorporated herein in its entirety.

BACKGROUND OF THE INVENTION

The subject matter disclosed herein relates to closed system heat engine, and in particular to a heat engine providing for unidirectional flow of a working gas and for controlling gas pressure within the heat engine.

External heat engines refer to several cycles that extract heat from an external source, such as solar or waste heat from a furnace, and deliver it to a working fluid inside the engine to produce useful work. Stirling cycle and Ericson cycles are some of the cycles that utilize external heat sources. Both cycles use regenerators, although the Stirling cycle is a closed system while the Ericson cycle is an open system. To date Stirling cycle engines have achieved the highest efficiency for a simple cycle due to the use of the regenerator that recovers heat that would normally be rejected from one cycle and delivers the thermal energy to the working gas in a subsequent cycle.

Stirling engines have traditionally been operated at high temperatures (750° C.) and high speeds (30-50 Hz), in an attempt to achieve high power to weight ratios. High temperature Stirling engines suffer from premature failure due to creep stress from high operating temperatures and pressures even when using exotic metals alloys, such as Inconel for example. Recent developments in Stirling engines have been directed to utilizing low temperature heat sources such as industrial waste heat, solar or geothermal for conversion into electricity. However, very few successful engines have been demonstrated and only at small power output levels.

Traditional Stirling engines operate by oscillating a working gas between a hot and cold heat exchanger and the expansion and compression pistons. The major deficiency of oscillating flow engines is that the hot and cold heat exchangers have a volume that is tied to the swept volume of the piston. In one half of the cycle it is necessary to compress and expand from this volume. Although during the other half of the cycle, it is considered dead space that has an adverse affect. The compression piston compresses the gas into the cold heat exchanger in order to achieve an isothermal compression (constant temperature). Further motion of the compression piston displaces some of the cold gas up through the regenerator and into the hot heat exchanger. The gas in the hot heat exchanger heats up while the expansion piston starts to expand. As such the gas expands from the hot heat exchanger volume in an isothermal expansion. The deficiency in this arrangement is that a large amount of working gas mass is left in the cold heat exchanger volume while expansion is occurring.

In high temperature engines leaving working gas in the cold heat exchanger is acceptable due to the volume ratio of the heat exchanger to the swept volume of the piston. In high temperature engines this is typically 1:3 but in low temperature engines this can be as low as 1.25:1. This means that in low temperature engines the heat exchanger volume is much larger than the swept volume and an excessive amount of gas is left in the cold heat exchanger. In effect more work is put in to compressing a gas that does not make it to the hot side of the engine to deliver useful work. This is the primary reason why traditional oscillating Stirling engines usually do not effectively operate below 300° C.

While existing heat engines are suitable for either intended purposes there remains a need for improvements in closed cycle heat engines to provide high efficiency operation at low engine temperatures.

BRIEF DESCRIPTION OF THE INVENTION

According to one aspect of the invention, a closed cycle heat engine is provided. The heat engine includes a first expansion piston arranged in a first cylinder. A second expansion piston is arranged in a second cylinder and operably coupled to the first expansion piston, wherein the second cylinder is fluidly coupled to the first cylinder. A compression piston is arranged in a third cylinder and operably coupled to the first expansion piston, wherein the third cylinder is fluidly coupled to received a working fluid from the second cylinder and transfer the working fluid to the first cylinder.

According to another aspect of the invention, a heat engine having a working fluid is provided. The heat engine includes a first set of cylinders having a first expansion piston fluidly and operably coupled to a second expansion piston. A first compression piston is fluidly and operably coupled to the first expansion cylinder and the second expansion cylinder. Wherein the first set of cylinders is arranged in a first closed cycle to provide unidirectional flow of the working fluid.

According to yet another aspect of the invention, a method of operating a closed cycle heat engine is provided. The method includes heating a working fluid. Expanding the heated working fluid into a first cylinder to move a first expansion piston to a first position. The heated working fluid is flowed into a second cylinder to move a second expansion piston to a second position. The working fluid is flowed from the first cylinder and the second cylinder to the third cylinder. The working fluid is compressed in the third cylinder with the compression piston. The working fluid is flowed from the third cylinder to the first cylinder.

According to yet another aspect of the invention, a heat engine is provided. The heat engine includes at least two expansion pistons fluidly coupled to each other, each expansion piston being arranged in an expansion cylinder. At least one compression piston is fluidly coupled to the at least two expansion pistons, each compression piston being arranged in a compression cylinder. A drive linkage is operably coupled to the at least two expansion pistons and the at least one compression piston, the drive linkage arranged in a crankcase, wherein the crankcase is fluidly coupled to the expansion cylinders and the compression cylinders. A pressurized vessel is also provided that is fluidly coupled to the crankcase, wherein the pressurized vessel is arranged to add pressurized gas to the crankcase in response to a change in the operation of the heat engine.

According to yet another aspect of the invention, a closed cycle heat engine is provided. The closed cycle heat engine includes a first expansion piston arranged in a first cylinder. A second expansion piston arranged in a second cylinder and operably coupled to the first expansion piston, wherein the second cylinder is fluidly coupled to the first cylinder. A compression piston is arranged in a third cylinder and operably coupled to said first expansion piston, wherein the third cylinder is fluidly coupled to received a working fluid from the second cylinder and transfer the working fluid to the first cylinder. A drive linkage is coupled to the first piston, the second piston and the third piston, wherein the drive linkage is a cam plate or an articulated crankshaft. A drive case is coupled to the drive linkage, the drive case being operated at a pressure less than or equal to a minimum pressure of the working fluid that occurs in the first cylinder and the second cylinder during operation of the engine. A regenerator is arranged to transfer thermal energy from the working fluid flowing from the second cylinder to the third cylinder to the working fluid flowing from the third cylinder to the first cylinder, wherein the regenerator is a counter flow heat exchanger. A heater thermally coupled between the regenerator and the first cylinder. A cooler is thermally coupled between the third cylinder and the regenerator. A first valve fluidly coupled between the heater and the first cylinder. A second valve is fluidly coupled between the second cylinder and the regenerator. A first check valve is fluidly coupled to the third cylinder to allow the working fluid to flow into the third cylinder. A second check valve is fluidly coupled to the third cylinder to allow the working fluid to flow from the third cylinder.

According to yet another aspect of the invention, a heat engine having a working fluid is provided. A heat engine includes a first set of cylinders having a first expansion piston fluidly and operably coupled to a second expansion piston and a first compression piston fluidly and operably coupled to the first expansion cylinder and the second expansion cylinder, wherein the first set of cylinders is arranged in a first closed cycle to provide unidirectional flow of the working fluid, wherein the first set of cylinders and said second set of cylinders are arranged 90 degrees apart. A second set of cylinders is operably coupled to the first set of cylinders, the second set of cylinders having a third expansion piston fluidly and operably coupled to a fourth expansion piston and a second compression piston fluidly and operably coupled to the third expansion cylinder and the fourth expansion cylinder, wherein the second set of cylinders is arranged in a second closed cycle to provide unidirectional flow of the working fluid. A regenerator is thermally coupled to the first closed cycle and the second closed cycle, wherein the regenerator is arranged in the portion of the first closed cycle between the second expansion piston and the first compression piston, and is arranged in the portion of the second closed cycle between the fourth expansion piston and the second compression piston. A heater is coupled to the first closed cycle and the second closed cycle to transfer thermal energy to the working fluid. A cooler is coupled to the first closed cycle and the second closed cycle to transfer thermal energy from the working fluid. A drive case is operably coupled to the first set of cylinders and the second set of cylinders, the drive case having an operating pressure less than or equal to a minimum pressure of the working fluid in the first cylinder and the second cylinder during the operation of the heat engine.

According to yet another aspect of the invention, a method of operating a closed cycle heat engine is provided. The method includes the steps of heating a working fluid. Expanding the heated working fluid into a first cylinder to move a first expansion piston to a first position. Flowing the heated working fluid into a second cylinder to move a second expansion piston to a second position. Flowing the working fluid from the first cylinder and the second cylinder to the third cylinder. Compressing the working fluid in the third cylinder with the compression piston. Flowing the working fluid from the third cylinder to the first cylinder. Transferring thermal energy from the working fluid flowing from the second cylinder to the third cylinder to the working fluid flowing from the third cylinder to the fourth cylinder before the step of heating the working fluid. Cooling the working fluid before the step of compressing the working fluid in the step where the heated working fluid is flowed into the second cylinder. Dwelling the second expansion piston at the second position when the working fluid flows from the first cylinder to the third cylinder. Finally a drive case is pressurized to a first pressure less than or equal to a pressure of the working fluid in the first cylinder and the second cylinder when the second expansion piston is in the second position.

These and other advantages and features will become more apparent from the following description taken in conjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWING

The subject matter, which is regarded as the invention, is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which:

FIG. 1 is a schematic block diagram illustration of a heat engine in accordance with an embodiment of the invention;

FIG. 2 is a schematic block diagram illustration of the heat engine of FIG. 1 with the first expansion piston in the bottom dead center position;

FIG. 3 is a schematic block diagram illustration of the heat engine of FIG. 1 with the second expansion piston in the bottom dead center position;

FIG. 4 is a schematic block diagram illustration of the heat engine of FIG. 1 during the adiabatic compression of the working fluid by the compression piston and the first expansion piston at the top dead center position;

FIG. 5 is a schematic block diagram illustration of the heat engine of FIG. 1 with the first and second expansion pistons at the top dead center position;

FIG. 6 is an illustration of an exemplary pressure-volume diagram for the thermodynamic cycle of the heat engine of FIGS. 1-5.

FIG. 7 is a plan view illustration of an alternate embodiment heat engine having eight expansion pistons;

FIG. 8 is another plan view illustration of the heat engine of FIG. 7;

FIG. 9 is a schematic illustration of the heat engine of FIG. 7;

FIG. 10 is a schematic illustration of another embodiment heat engine having crankcase pressure control;

FIG. 11 is a partial perspective view illustration of an alternate embodiment heat engine having a crank arrangement;

FIG. 12 is a side plan view illustration of the heat engine of FIG. 11;

FIG. 13 is a partial perspective view illustration of an alternate embodiment heat engine having six sets of cylinders; and,

FIG. 14 is a top plan view illustration of the heat engine of FIG. 13.

The detailed description explains embodiments of the invention, together with advantages and features, by way of example with reference to the drawings.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIGS. 1-5 and exemplary embodiment of an external heat engine 20 is shown. The heat engine 20 utilizes an external heat source 22 to deliver heat energy to a working gas in the heat engine 20. The heat engine 20 is a closed system that incorporates a hot heat exchanger 24, a cold heat exchanger 26, and a regenerator 28. The heat engine 20 further includes a housing 30 that includes at least three of cylinders 34, 36, 38. The first or compression cylinder 34 contains a compression piston 32, for compressing the cold working gas. The compression piston 32 is coupled to two expansion pistons 40, 42 that are contained within expansion cylinders 36, 38 respectively. Each of the pistons is coupled to a drive linkage 44. The heat engine 20 also includes valves 46, 48, 50, 52 to control the flow of the working gas through the heat engine 20.

The heat engine 20 utilizes a unidirectional flow of the working gas from the compression cylinder 34 through the regenerator 28 to the hot heat exchanger 24 and to the expansion cylinders 36, 38 and back again. As will be made clearer herein, the unidirectional flow of the gas provides advantages by causing substantially all of the compressed working gas through the hot heat exchanger 24, since all of the working gas has to propagate through the heat engine 20 in one direction. The heat engine 20 also provides advantages by utilizing adiabatic compression followed by an isobaric heating process in the hot heat exchanger 24.

Referring now to FIG. 1, the first expansion piston 40 has an intake valve 46 which opens and allows high pressure (Pmax), high temperature (Tmax) working gas to flow into the first expansion cylinder 36 as the first expansion piston 40 moves. Since the compression piston 32 and the first expansion piston 40 move together there is a small overall volume change in this first expansion of the working gas. The effect is that the pressure and temperature of the working gas remains substantially constant which resembles an isobaric/isothermal expansion. Once the first expansion piston 40 reaches the bottom dead center position shown in FIG. 2, the intake valve 46 closes and the second expansion piston 42 that was dwelling at top dead center position is allowed to move. As will be explained in more detail herein, the movement of the pistons 32, 40, 42 are defined by the drive linkage 44. In the exemplary embodiment, the drive linkage 44 is a cam plate having at least one profile.

The volume in the first expansion cylinder 36 makes up the initial expansion volume for the second expansion cylinder 38. The expansion of the working gas in the second expansion cylinder 38 occurs substantially adiabatically. FIG. 3 illustrates that while the second expansion piston 42 is moving, the first expansion piston 40 is dwelling at the bottom dead center position. After the second expansion piston 42 reaches the bottom dead center position, the working gas will have expanded to the minimum pressure (Pmin) of the heat engine 20.

After the second expansion piston 42 reaches the bottom dead center position, an exhaust valve 48 opens to allow displacement of the working gas towards the cold heat exchanger 26. It should be appreciated that the heat engine 20 may be arranged, through the configuration of the drive linkage 44 for example, to displace the working gas from the expansion cylinders 36, 38 either simultaneously, or sequentially as is shown in FIG. 4. When the working gas is displaced from the expansion cylinders 36, 38 it still contains a substantial amount of thermal energy. The working gas first passes through the regenerator 28, which extracts the thermal energy of the working gas flowing towards the cold heat exchanger 26. In the exemplary embodiment, the regenerator 28 is a counter flow heat exchanger, which can be a multiple plate or tube in tube type exchanger where hot gas is flowing in one direction through a chamber and cold gas is flowing in the opposite direction in an adjacent chamber. The primary function of regenerator 28 is to extract heat from the exhaust working gas (i.e. the working gas flowing toward the cold heat exchanger 26) before it gets to the cold heat exchanger 26 and deliver it to the cold compressed working gas flowing from the compression cylinder 34 before it gets to the hot heat exchanger 24.

After the working gas passes through the regenerator 28 it then goes to the cold heat exchanger 26 where substantially all of the non-recoverable thermal energy is extracted and rejected to the atmosphere through a radiator 54. The cold working gas leaving the cold heat exchanger 26 then enters the compression cylinder 34 where the compression piston 32 compresses the working gas adiabatically, as is shown in FIG. 5. Once the pressure in the compression cylinder 34 reaches a maximum pressure (Pmax) a check valve 50 opens and allows the compressed working gas to be displaced out of the compression cylinder 34 towards the regenerator 28. A second check valve 52 is arranged adjacent to the inlet to the compression cylinder 34 to prevent flow back towards the cold heat exchanger 26. The cold working gas flowing through the regenerator 28 is heated by the exhaust working gas flowing in the opposite direction as discussed above, and leaves the regenerator 28 at a temperature below the maximum operating temperature (Tmax). The working gas then passes through the hot heat exchanger 24 where thermal energy from an external source is once again used to heat the working gas to the maximum temperature (Tmax) and the thermodynamic cycle repeats.

Another way of describing the operation of the heat engine 20 is with a pressure-volume (“PV”) diagram 100 as illustrated in FIG. 6. A PV diagram is typically used to describe thermal cycles. The PV diagram 100 is useful in the calculation of the amount of work done by the heat engine 20, which is the integral of the pressure with respect to volume. The work can be quickly calculated from the PV diagram 100 by determining the area enclosed by the cycle. In general, when the operation of the heat engine moves clockwise about the PV diagram, positive work is created.

The cycle starts at point 102, which corresponds to the position of the heat engine 20 in FIG. 1. At this point, the heat engine 20 is at its minimum pressure and volume. As the hot working gas is allowed to pass through the intake valve 46 into the expansion cylinder 36, the pressure in the heat engine 20 increases while maintaining the volume substantially constant. Once the maximum pressure has been reached, the intake valve 46 closes and the cycle is at point 104, which corresponds to FIG. 2.

The drive linkage 44 is arranged to allow the second expansion piston 42 to then move, thereby allowing the working gas to expand into the second expansion cylinder 38. This allows the volume of the heat engine 20 to increase, while also decreasing the pressure as the cycle moves from point 104 to point 106. After reaching point 106, the exhaust valve 48 is opened, allowing the working gas to flow from the expansion cylinders 36, 38, through the regenerator 28 and cold heat exchanger 26, and into the compression cylinder 34. It should be appreciated that while FIGS. 4 and 5 illustrate the expansion pistons 40, 42 moving serially, the drive linkage 44 may also be arranged to move both expansion pistons 40, 42 simultaneously. As the working gas flows from the expansion cylinders 36, 38 to the compression cylinder 34, the pressure drops and the volume increases until the cycle reaches point 108, which corresponds to the position of the heat engine 20 shown in FIG. 5.

To finish the thermodynamic cycle, the cold working gas flows from back from the compression cylinder 34, absorbing heat in the regenerator 28 and hot heat exchanger 24. As the heat engine 20 returns to the position shown in FIG. 1, the volume drops while the pressure is maintained at a nearly constant level. As discussed above, since the cycle illustrated in FIG. 6 moves in a clockwise direction, the heat engine 20 creates positive or useful work.

An alternate embodiment heat engine 200 is shown in FIGS. 7-10. In this embodiment, the heat engine 200 includes four sets of pistons 202. Each set of pistons 202 includes a compression piston 32, a first expansion piston 40 and a second expansion piston 42. Each set of pistons 202 individually operates in the same thermodynamic cycle as described herein above with respect to FIGS. 1-5. The sets of pistons 202 are arranged radially about center axis 204 of the engine housing 206 such that the sets of pistons are 90° apart.

As best seen in FIG. 8 and FIG. 9, the heat engine 200 includes a drive linkage 208, such as a cam plate for example, having at least a first cam profile 210 and a second cam profile 212. In this embodiment, the motion of the first expansion piston 40 and the compression piston 32 are determined by followers 214, 216 that engage the first cam profile 210. Similarly, the second expansion piston 42 has a follower 218 that engages the second cam profile 212. The cam profiles 210, 212 are arranged to have geometry suitable to provide the timing and motion of pistons 32, 40, 42 to facilitate the thermodynamic cycle illustrated in FIG. 6. In this embodiment, the followers 214, 216, 218 include a tie rod and a roller. Other components (not shown) may also be utilized in maintaining the followers 214, 216, 218 engaged with the drive linkage 208 as is known in the art. It should be appreciated that in other embodiments, the heat engine 200 includes a third cam profile for the compression piston 32, such that compression piston 32 is not in phase with the expansion pistons 40, 42. It should further be appreciated that the drive linkage 208 may also be in the form of a camshaft instead of a cam plate.

Referring now to FIG. 9, it can be seen that each of the sets of pistons 202 are in a different phase of the thermal cycle of FIG. 6. This allows the heat engine 200 to operate smoothly at a substantially constant revolutions-per-minute (“RPM”). Each of the expansion pistons 40, 42 are fluidly coupled to valves 46, 48 as described herein above. Opposite each of the first expansion pistons 40, the intake valves 46 are coupled together by a manifold to a single conduit 220 that allows each of the first expansion pistons 40 to be coupled to a single hot heat exchanger 24. Similarly, opposite each of the second expansion pistons 42, the exhaust valves 48 are coupled together by a manifold to a conduit 222 that provides a single connection to the regenerator 28.

The regenerator 28 is coupled to a conduit 224 that connects the regenerator 28 to the cold heat exchanger 26. The cold heat exchanger 26, in turn is coupled together by a manifold a single conduit 226 that distributes the working gas to the compression cylinders 34 via second check valve 52. After compressing the working gas, each of the compression cylinders 34 is coupled to a manifold 228 via check valves 50. A conduit 230 couples the manifold 228 to the regenerator 28. The regenerator 28 in turn is coupled to the hot heat exchanger 24 by conduit 232 to complete the fluid circuit.

Another embodiment of a heat engine 300 is illustrated in FIG. 10. Similar to the embodiments described above, the heat engine 300 includes one or more sets of pistons 302 having a first expansion piston 40, a second expansion piston 42 and a compression piston 32. The heat engine 300 also includes a hot heat exchanger 24, a cold heat exchanger 26 and a regenerator 28 as described herein above.

The expansion pistons 40, 42 and the compression piston 32 are operably coupled to a drive linkage 304 positioned within a crankcase 306. Fluidly coupled to the crankcase 306 is a first high pressure vessel or tank 308 (e.g. 2000 psi) and a second low-pressure vessel or tank 310 (e.g. 100 psi). The tanks 308, 310 are arranged to add and subtract gas from the heat engine 300 depending on the desired operating speed. It should be appreciated that the tanks 308, 310 may include one or more components (not shown), such as valves 312, pumps 314 and sensors 316 for example, to facilitate the flow of gas. In one embodiment, the pressure in the crankcase 306 is maintained at or less than the minimum pressure realized by the heat engine during the thermodynamic cycle, such as the pressure at points 102, 108 on the PV diagram of FIG. 6. In one embodiment, the pressure sensor 316 measures the pressure in conduit 318 between the cold heat exchanger 26 and the compression cylinder 34. By adjusting the pressure of the crankcase 306 at this level, any leakage of the working gas from the cylinders will flow into the crankcase 306 where it can be recycled. The maintaining of pressure in the crankcase 306 also provides advantages in that it also minimizes the differential pressure across the piston seals (not shown) reducing the force on the drive linkage 304.

The tanks 308, 310 are also coupled to conduit 320 between the regenerator 28 and the compression piston 32. A pump 324 is coupled between the tanks 308, 310 to transfer working gas from the low-pressure tank 310 to the high-pressure tank 308. It should be appreciated that the conduits connecting the tanks 308, 310 to the crankcase 306 and conduit 320 are illustrated as being separately connected to the tanks 308, 310, however, it is contemplated that other arrangements may also be utilized.

During operation, the tanks 308, 310 are arranged to remove working gas from the conduit 320 into low-pressure tank 310 when power needs to be reduced. Once the valve that connects low-pressure tank 310 to conduit 320 is closed, the pump 314 transfers the working gas to high-pressure tank 308. As the demand for power increases, working gas flows from high-pressure tank 308 back into conduit 320. This provides an advantage in maintaining the heat engine 300 at a constant RPM while varying the output power. This also provides advantages in balancing the amount of working gas in the heat engine 300 at any given time.

Another embodiment heat engine 350 is illustrated in FIGS. 11-12. Similar to the embodiments described above, the heat engine has a plurality of cylinders arranged in a housing 30. It should be appreciated that the heat engine 350 also includes a regenerator 28, heat exchangers 24, 26 and valves 46, 48, 50, 52 as described herein, however these components are not illustrated in FIGS. 13-14 for purposes of clarity. Alternately, heat engine 350 may also include tanks 308, 310 as described herein. The heat engine 350 further includes a set of pistons including first expansion piston 40, second expansion piston 42 and compression piston 32. Each piston 32, 40, 42 has an associated connector rod 352. Unlike the embodiments described above, in this embodiment the compression piston 32 and second expansion piston 42 are coupled to a first drag link 354. The first drag link 354 pivots about a shaft 356. The coupling of the second expansion piston 42 and the compression piston 32 provides advantages in allowing the use of a drive linkage 44 that includes a crank assembly 362 where the second expansion piston 42 can dwell at the top-dead-center position and the compression piston 32 can dwell at the bottom-dead-center position. The first expansion piston 40 is coupled to a second drag link 358, which pivots on a shaft 360.

The drive linkage 44 includes a top linkage 364, a bottom linkage 366 and a crank arm 368 which are coupled for rotation about a pin 370. The bottom linkages 366 are coupled to the drag link 354 adjacent the connector rod 352 for the first expansion piston 40 and the compression piston 32. The top linkage 364 pivots about a shaft 372 that fixes the end of the top linkage 364 to the crankcase 306. Each crank arm 368 in turn is coupled one of the lobes 374 in the crank assembly 362. It should be appreciated that the motion of the pistons 32, 40, 42 is defined by the linkages 354, 366, the crank arm 368 and the trajectory of the connection point on lobes 374. In this embodiment, the motion of the expansion pistons 40, 42 is arranged such that the working fluid is exhausted from both expansion cylinders 36, 38 simultaneously. The crank assembly 362 further includes a drive shaft 376 that transfers the rotational movement generated by the heat engine 350. It should be appreciated that the heat engine 350 also includes bearing housings that are not shown for purposes of clarity.

In this embodiment, the thermodynamic operating cycle is defined by the operating modes of the compression piston 32 and expansion pistons 40, 42. The cycle starts by adiabatically compressing a cold working fluid. The working fluid then passes through a heat exchanger, such as hot heat exchanger 24 (FIG. 1) for example, where it is heated. Valve 46 (FIG. 1) over the first expansion piston 40 opens to allow the hot working fluid to flow into the first expansion cylinder 36. While the first expansion piston 40 is moving down the working fluid is expanding isothermally (working fluid pressure reduces while the temperature stays constant). Once the first expansion piston 40 reaches the bottom of travel path, the valve 46 that was opened then closes. The volume of the working fluid within first expansion cylinder 36 becomes the initial expansion volume for the working fluid in the second expansion cylinder 38. While the second expansion piston 42 is moving down the working fluid is expanding adiabatically (e.g. both the pressure and temperature of the working fluid will be reduced). Once the second expander piston 42 reaches the bottom of its travel path, the exhaust valve 48 (FIG. 1) opens and both the expansion pistons 40, 42 substantially simultaneously exhaust the lower pressure and temperature working fluid from the expansion cylinders 36, 38 to the regenerator 28. While the exhaust working fluid passes through the regenerator 28, heat is transferred to the cold compressed working fluid that is flowing from the compression cylinder 34 to the hot heat exchanger 24. After leaving the regenerator 28, any residual heat remaining in the working fluid is rejected through a cold heat exchanger 26 by an appropriate medium, such as cooling water for example. The cold working fluid is then in a position to start the cycle again.

In this embodiment, the second expansion piston 42 has a smaller cross-sectional area than the first expansion piston 36. The sizing of the expander pistons 40, 42 and compression piston 32 may be determined by the temperature differential between the heating source 22 (FIG. 1) and the cold medium that removes heat from the radiator 54 (FIG. 1). The cold medium is typically the atmosphere (e.g. atmospheric air temperature), but alternately may use other types of mediums such as river water or even liquid propane for example. The expansion ratio is defined as T_(hot)/T_(cold). For example, if the hot side is 700 K and the cold side is 300 K then the expansion volume ratio is 2.333:1. In this example, the ratio between the initial expansion volume in the first expansion cylinder 36 and the total expansion volume in both expansion cylinders 36, 38 is large. For high temperature differentials between the hot heat exchanger 24 and the cold heat exchanger 26, the second expansion piston 42 will have a larger surface area than the first expansion piston 40. For low temperature differentials, such as for steam condensing, the ratio is approximately 1:1.25. In these low temperature differentials, the hot side may be between 370 K and 425K with the cold side being at atmospheric temperature. In these embodiments, the first expansion piston 40 will be larger than the second expansion piston 42. Further, the piston diameters and stroke lengths may be determined by the desired amount of power generated at a specific frequency. The higher the frequency, the smaller the expansion pistons 40, 42 will be for a given power. In some embodiments, the desired operating frequency is between 5-10 Hz. Operation of the heat engine at this frequency provides advantages in diminishing flow losses and maintenance issues associated with wear items.

Another embodiment heat engine 380 is illustrated in FIGS. 13-14. In this embodiment, the heat engine 380 is comprised of multiple sets of pistons 382 each of which are substantially the same as the embodiment described with reference to FIGS. 11-12. It should be appreciated that the heat engine 380 also includes a regenerator 28, heat exchangers 24, 26 and valves 46, 48, 50, 52 as described herein, however these components are not illustrated in FIGS. 13-14 for purposes of clarity. Alternately, heat engine 380 may also include tanks 308, 310 as described herein. Each set of pistons 382 includes a compression piston 32, a first expansion piston 40, and a second expansion piston 42. The compression piston 32 and the second expansion piston 42 are coupled by a drag link 354 as described above Each set of pistons 382 is coupled by a drive linkage 44 to a crank assembly 384 having a crankshaft 386. The crankshaft 386 couples each of the sets of pistons 382 to transfer the power generated during operation to an external process, such as an electrical generator for example. It should be appreciated that the sets of pistons 382 cooperate to operate in sequence to provide substantially continuous rotation of the crankshaft 386.

It should be appreciated that while the embodiments described herein refer to a heat engine having a single compression piston and cylinder, however the claimed invention should not be so limited. In other embodiments, the heat engine may have multiple compression cylinders while still providing the unidirectional flow of the working fluid. In one embodiment, the working fluid is divided between two compression cylinders.

The heat engine disclosed herein provides a number of advantages over oscillating flow engines of the prior art. The heat engine eliminates the problem of mass transfer efficiency in that all of the working gas that is compressed gets to the hot side of the heat engine to deliver useful work. Utilizing unidirectional flow a counter flow heat exchanger instead of the traditional regenerators used in oscillating flow engines provides another advantage. Counter flow heat recovery is more efficient than oscillating flow regenerators due to the ability of counter flow heat exchangers to maintain a higher thermal gradient across the length of the regenerator.

While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims. 

1. A closed cycle heat engine comprising: a first expansion piston arranged in a first cylinder; a second expansion piston arranged in a second cylinder and operably coupled to said first expansion piston, wherein said second cylinder is fluidly coupled to said first cylinder; a compression piston arranged in a third cylinder and operably coupled to said first expansion piston, wherein said third cylinder is fluidly coupled to received a working fluid from said second cylinder and transfer said working fluid to said first cylinder.
 2. The heat engine of claim 1 further comprising a drive linkage coupled to said first expansion piston, said second expansion piston and said compression piston.
 3. The heat engine of claim 2 wherein said drive linkage is a cam plate.
 4. The heat engine of claim 2 wherein said drive linkage is an articulated crankshaft.
 5. The heat engine of claim 2 further comprising: a drive case coupled to said drive linkage, said drive case being operated at a pressure less than or equal to a minimum pressure of said working fluid that occurs in said first cylinder and said second cylinder during operation of said heat engine.
 6. The heat engine of claim 1 further comprising a regenerator arranged to transfer thermal energy from said working fluid flowing from said second cylinder to said third cylinder to said working fluid flowing from said third cylinder to said first cylinder.
 7. The heat engine of claim 6 wherein said regenerator is a counter flow heat exchanger.
 8. The heat engine of claim 6 further comprising: a heater thermally coupled between said regenerator and said first cylinder; and, a cooler thermally coupled between said third cylinder and said regenerator.
 9. The heat engine of claim 8 further comprising: a first valve fluidly coupled between said heater and said first cylinder; a second valve fluidly coupled between said second cylinder and said regenerator; a first check valve fluidly coupled to said third cylinder to allow said working fluid to flow into said third cylinder; and a second check valve fluidly coupled to said third cylinder to allow said working fluid to flow from said third cylinder.
 10. A heat engine having a working fluid comprising: a first set of cylinders having a first expansion piston fluidly and operably coupled to a second expansion piston and a first compression piston fluidly and operably coupled to said first expansion piston and said second expansion piston, wherein said first set of cylinders is arranged in a first closed cycle to provide a first unidirectional flow of said working fluid.
 11. The heat engine of claim 10 further comprising: a second set of cylinders operably coupled to said first set of cylinders, said second set of cylinders having a third expansion piston fluidly and operably coupled to a fourth expansion piston and a second compression piston fluidly and operably coupled to said third expansion piston and said fourth expansion piston, wherein said second set of cylinders is arranged in a second closed cycle to provide a second unidirectional flow of said working fluid.
 12. The heat engine of claim 11 wherein said first set of cylinders and said second set of cylinders are arranged 90 degrees apart.
 13. The heat engine of claim 12 further comprising a regenerator thermally coupled to said first closed cycle and said second closed cycle.
 14. The heat engine of claim 13 further comprising a heater coupled to said first closed cycle and said second closed cycle to transfer thermal energy to said working fluid.
 15. The heat engine of claims 14 further comprising a radiator coupled to said first closed cycle and said second closed cycle to transfer thermal energy from said working fluid.
 16. The heat engine of claim 15 wherein said regenerator is arranged in a first portion of the first closed cycle between the second expansion piston and the first compression piston, and is arranged in a second portion of the second closed cycle between the fourth expansion piston and the second compression piston.
 17. The heat engine of claim 11 further comprising a drive case operably coupled to said first set of cylinders and said second set of cylinders, said drive case having an operating pressure less than or equal to a minimum pressure of said working fluid in said first set of cylinders and said second set of cylinders during operation of said heat engine.
 18. A method of operating a closed cycle heat engine comprising: heating a working fluid; expanding said heated working fluid into a first cylinder to move a first expansion piston to a first position; flowing said heated working fluid into a second cylinder to move a second expansion piston to a second position; flowing said working fluid from said first cylinder and said second cylinder to a third cylinder; compressing said working fluid in said third cylinder with a compression piston; and, flowing said working fluid from said third cylinder to said first cylinder.
 19. The method of claim 18 further comprising transferring thermal energy from said working fluid flowing from said second cylinder to said third cylinder to said working fluid flowing from said third cylinder to said first cylinder before said step of heating said working fluid.
 20. The method of claim 19 further comprising cooling said working fluid before said step of compressing said working fluid in said third cylinder.
 21. The method of claim 20 further comprising dwelling said first expansion piston at said first position during said step of flowing said heated working fluid into said second cylinder.
 22. The method of claim 21 further comprising the step of dwelling said second expansion piston at said second position during said flowing of said working fluid from said first cylinder to said third cylinder.
 23. The method of claim 18 further comprising: adjusting a drive case to a first pressure less than or equal to a pressure of said working fluid in said first cylinder and said second cylinder when said second expansion piston is in said second position.
 24. A heat engine comprising: at least two expansion pistons fluidly coupled to each other, each expansion piston being arranged in an expansion cylinder; at least one compression piston fluidly coupled to said at least two expansion pistons, each compression piston being arranged in a compression cylinder; a drive linkage operably coupled to said at least two expansion pistons and said at least one compression piston, said drive linkage arranged in a crankcase, wherein said crankcase is fluidly coupled to said expansion cylinder and said compression cylinder; and, a pressurized vessel fluidly coupled to said crankcase, wherein said pressurized vessel is arranged to add pressurized gas to said crankcase in response to a change in operation of said heat engine. 